Inkjet printing processes fall into two main types: continuous processes and drop-on-demand (DOD) processes. Continuous processes use electrically conductive inks to produce a stream of drops electrically charged ink that are deflected by an electric field to an appropriate location on a substrate. In DOD processes, individual drops of ink are expelled from the nozzle of a print head either by vibration of a piezoelectric actuator (in piezoelectric inkjet printing) or by heating the ink to form a vapourised gas bubble (in thermal inkjet printing, also known as bubblejet printing). Thermal inkjet printing has advantages over piezoelectric printing, with printers and print heads being lower cost and with the printing process being able to achieve better resolution.
Inkjet inks need to satisfy a number of requirements, including the following:
1. Viscosity must be appropriate. With DOD inks there are greater limitations on inks for thermal printing than for piezoelectric printing, with it generally being necessary for inks to have a viscosity of below 4 mPa·s at printhead operating temperature (typically 35 to 85° C.), which usually equates to a viscosity of less than 12 mPa·s at room temperature (25° C.), to be capable of being thermally inkjet printed. In this specification, all viscosity values are at 25° C. unless otherwise specified.
2. The ink must not cause unacceptable levels of clogging or blockage of printing nozzles.
3. The ink must not result in build up of deposits on the ejection heaters of thermal inkjet print heads (a process known as kogation) to an unacceptable level during the working life of a printhead.
4. The ink should be stable in storage, without settling out or coagulation of materials.
5. The resulting print needs to satisfy desired characteristics depending on the field of use, with possible relevant factors including water fastness, scratch resistance, durability, lack of shrinkage, lack of cracking, flexibility, optical density (for coloured inks), uniformity of deposition.
Conventional thermal inkjet printheads typically require use of an ink with a high percentage (over 50% by weight) of water or volatile organic solvent in order to achieve effective inkjet printing. While generally a poor solvent for organic compounds and having too high a surface tension to wet many plastic substrates, water may nevertheless be the ideal solvent for coating and ink delivery, being able to lower viscosity and volatilize without adding to emissions, toxicity or odour. Aqueous inks, however, require proportionally longer drying times than typical solvent-based inks, thus impacting on the maximum rate of printing.
Increasing the print speed often means that the inkjet ink should desirably dry more quickly such that the final print does not smudge when stacked. Inks demonstrating reduced drying times can be achieved by, for example, using volatile solvents in place of at least some of the water in the liquid medium. However solvent-based inks are potentially hazardous due to flammability issues and environmental concerns.
Inks with a quicker drying time due to lower water content, however, often result in an increased tendency for the ink to dry in the nozzles whilst the printer is not actively printing. Ink dried in nozzles tends to cause difficulty in printing properly again after periods of rest. Nozzles may become blocked, may fire intermittently or may fire improperly, a kind of operability problem more specifically referred to as decap.
Solvents such as alcohol or methyl ether ketone (MEK) are known to improve (shorten) drying time, since such substances have higher vapour pressure than water, and so evaporate faster. Also, these solvents have a surface tension less than 25 dynes/cm, compared to 72 dynes/cm for water, giving the solvents superior wetting characteristics on non-porous substrates, and helping adhesion to the surface.
Various inks for inkjet printing processes are known in the art. Generally, the inkjet inks used in the art are aqueous inks, comprising a major quantity of water, a humectant and/or a co-solvent, together with a colouring agent. By selecting specific surfactants, humectants, colouring agents, or other components, it is possible to adjust the print characteristics of the resultant ink.
Although numerous inkjet inks are presently available, they generally do not meet all of the above-described requirements, while also providing excellent print quality on the wide variety of plain papers generally used in the home and office. Particularly, adhesion of these inkjet inks to semi-porous and non-porous substrates is desirable.
Great effort has been expended in attempts to provide both dye-based and pigment-based inkjet inks having acceptable adhesion on non-porous substrates while maintaining other desirable characteristics. However, there continues to be a demand for inks having all of the above-mentioned desirable characteristics.
More recently, commercial interest in thermal inkjet inks has tended to focus more on developing curable ink formulations, more specifically radiation- or thermally-curable ink formulations, in which a UV (ultra violet) or infra red lamp, or a combination thereof, is employed to bring about a fast cure rate. The use of such devices necessarily reduces the time-consuming process for removing solvent from the ink thereby increasing the production speed, particularly in industrial printing processes such as in mailing and addressing.
U.S. Pat. No. 5,623,001 discloses UV-curable aqueous DOD inkjet inks, particularly for piezoelectric printing, comprising water and water-miscible UV-curable monomer and/or oligomer e.g. acrylic materials. The document makes no reference to thermal inkjet printing, and does not teach how to make inks, particularly low water content inks, suitable for thermal inkjet printing.
U.S. 2002/0198289 concerns UV-curable aqueous thermal inkjet inks comprising UV-curable resin, suitable for printing waterfast images on porous media. The examples all use a water content of over 35%, and there is no teaching of how to make a low water content thermal inkjet ink.
U.S. Pat. No. 4,978,969 concerns UV-curable thermal inkjet inks with good adhesion to plastics. The inks comprise UV-curable adhesive, and the exemplified inks use a mixture of three solvents, water, methyl ethyl ketone and gamma-butyrolactone, each at 5% by weight to ensure a homogenous inkjet ink composition with viscosity and surface tension properties suitable for use in inkjet printing. The benefits of organic solvent-based inkjet inks are overshadowed by the environmental and hazardous downsides of these components particularly the issues of flammability and transportation. Moreover, inkjet ink compositions comprising volatile liquids are often subject to an increased tendency for the ink to dry in the nozzles. The document does not teach how to make a single phase ink with a water content above 5% by weight.
WO 03/011989 describes a UV-curable ink for inkjet printing that is free of aqueous and volatile organic solvents.
WO 2004/092287 discloses essentially non-aqueous radiation-curable inkjet ink compositions.
WO 2006/102524 discloses substantially non-aqueous (i.e. having a water content of less than 10%) UV-curable thermal inkjet inks using a volatile driver fluid such as acetone, methyl or ethyl acetate, methanol, ethanol or propanol.
U.S. Pat. No. 6,790,875 discloses low viscosity curable thermal inkjet inks, with the exemplified inks generally having a water content in excess of 50% by weight.
U.S. Pat. No. 5,952,401 concerns low viscosity aqueous inkjet inks generally containing at least 50% by weight water.
Curable materials, e.g. monomers and oligomers, tend to have limited solubility/miscibility in water, and substantial practical difficulties arise in producing water-based inks with sufficiently low viscosity to be useful for thermal inkjet printing that do not undergo undesirable phase separation, do not cause clogging or blockage of printing nozzles, do not result in unacceptable levels of kogation, and that produce prints with appropriate, useful properties.
We have found that by use of mixtures of curable materials together with a water-compatible solvent (referred to as co-solvent) for the curable materials it is possible to produce low viscosity compositions suitable for thermal inkjet printing having a reduced water content compared to known aqueous thermal inkjet inks without needing to use undesirable volatile organic solvents while still being capable of achieving effective bubble nucleation.
The challenge therefore is to provide a single phase, low aqueous solvent containing radiation-curable thermal inkjet ink formulation that retains characteristics of good ejection stability, while removing the need for infra red and/or increased UV exposure to accelerate the drying process on non-porous media.